have you read his sects and sectarianism? it would help to understand his position more. according to his thesis the schism had to happen after asoka (if the edicts are right and the sangha was made unified, this means one sangha right? ) therefore any text from this era or before would be from a unified buddhism. he also doesnt paint the texts with the broad brush you ascribe to him. you are correct that he isnt writing from a standard academic approach, though that is not what he ever claims to be doing.
Yes, read it some time back. It is ok, but has a few problems. But we can leave that for another discussion. Needless to say, it is not saying much more than what has already been said by people like Bareau, Lamotte, Yinshun, and a host of Japanese scholars, decades ago. It just says it in a way for intelligent western practitioners of Theravada.
My understanding is that the schisms took place over some time, stretching from before Asoka, to well after Asoka.
The edict does not necessarily describe what actually happened, but like a lot of govt propaganda, may well simply indicate what Asoka wanted people to think had happened. See Schopen on the prescriptive vs descriptive nature of Buddhist material, both scriptural and epigraphic.
Likewise, too, the compilation of the canon into Agamas / Nikayas, and so forth, took place over some time. Greater parallels are found in the early strata, less in the later.
What we now have as the Pali Nikayas, and also the Chinese translations, have been reworked over at quite a later date, too. In particular, the Theravada Nikayas, whereas the Sarvastivada tend to leave in all sorts of "cut and paste" markers, and other traces.
So, in response to "therefore any text from this era or before would be from a unified buddhism" is out of the question. Firstly, we don't really have anything "from this ear or before". The parts that we think are closest also do not exhibit 100% coherence. So which is right? How can it be "from a unified Buddhism"? Moreover, the split between the Sthavira and Mahasamghika groups is before Asoka. So even Asokan period scripture (if we even had such a thing) would only be be Sthavira material, before splits into Vibhajjavada, Sarvastivada, Puggalavada, etc. As we do not even have any really early material from the Mahasamghikas, how do we know what texts they had? We cannot. So, we must hedge the range of any claims from restricted sources.
My comments viz "academic approach" were in response to the claims of an earlier poster in this thread, and not to claims of Ven Sujato himself.